More than a decade of research led to the conclusion in 2022 that the Soil Biome is home to ~ 2.1 × 10²⁴ taxa and thus supports > 99.9% of global species biodiversity, mostly Bacteria or other microbes, based upon topographic field data. A subsequent 2023 report tabulated a central value of just 1.04 × 10¹⁰ taxa claiming soils had 59 ± 15%, i.e., 44–74% (or truly 10–50%?) of the global total, while incidentally confirming upper values of ~ 90% for soil Bacteria. Incompatibility of these two studies is reviewed, supporting prior biodiversity data with the vast majority of species inhabiting soils, despite excluding viruses (now with ~ 5 × 10³¹ virions and 10²⁶ species most, ~ 80%, in soils). The status of Oligochaeta (earthworms) and other taxa marked “?” in the 2023 paper are clarified. Although biota totals are increased considerably, inordinate threats of topsoil erosion and poisoning yet pertain with finality of extinction. Species affected include Keystone taxa, especially earthworms and microbes, essential for a healthy Soil foundation to sustain the Tree-of-Life inhabiting the Earth.

Bacteria, earthworm, microbes, -Omics, soil organisms, species richness, viruses

Healthy soil is fundamental to sustainable existence of most species evolving on Earth in Darwin’s “Tree of Life” (a paradigm defended by Gulik et al. 2024). Soil supports more than 99.9% of species diversity and, now that vascular plants that seed and root in soils are included (Blakemore 2024), it supports 99% biomass hence ~ 98% of Net Primary Productivity (NPP) and also O₂ production. Bar-On and Milo (2019) had 0.7 Gt of photosynthetic/oxygenase Rubisco enzyme powering terrestrial environments (doubled for terrain to 1.4 Gt) with just ≈ 0.03 Gt (2.1%) in the marine environment. Soil filters and stores freshwater stocks (being subject to Earth tides!) and, as well as ~ 99% of human food, it provides most building materials plus many of our essential medicines/antibiotics. Thus, an important metric must be the scope and snapshot status of living or dormant Soil biota. A recent review by Anthony et al. (2023) claimed “two times greater soil biodiversity than previous estimates”, seemingly because Decaëns et al. (2006) had 23% “soil animals” in their tally of described species as known at that time (Fig. 1). Both assertions are challenged for several reasons, not least > 90–99% Soil biota reports by Williamson et al. (2017), Bickel and Or (2020), Zhao et al. (2022), and by Blakemore (2018b, 2022, 2023).

Figure 1. 

Decaëns et al. (2006: fig. 1) had “soil animals 23%” (i.e., ~ 360,000 in ~ 1.5 million species = 24%?).

Much higher totals had been determined since 2006, and Williamson et al. (2017) concluded: “Soils represent the greatest reservoir of biodiversity on the planet; prokaryotic diversity in soils is estimated to be three orders of magnitude greater than in all other ecosystems combined.” In other words, soils may contain 99.9% of species, mainly microbes. Supporting this were, for example, Bickel and Or (2020) or Zhao et al. (2022) who said: “soil is the most microbiologically abundant (10²⁹) and diverse (10¹¹) environment on the Earth” and, in their figure (Zhao et al. 2022: fig. 3A), these latter authors showed soil taxa at > 10× that of all aquatic species. In other words, > 90% of biodiversity is present in Soil vs Ocean. Independently, around the same time, Blakemore (2022) estimated the “Soil Realm” is home to ~ 2.1 × 10²⁴ taxa, or > 99.9% of global biodiversity, mostly Bacteria/Archaea or other microbes (excluding viruses), based upon published reports and extrapolation of topographic field data. Thus, rather than doubling it to ~ 50%, Anthony et al. (2023) actually halved soil biodiversity from > 99.9%. It is also remarkable that Decaëns et al.’s limited review claiming 23% biota seemed acceptable, unchallenged from 2006 because, instead of appraisal of realistic totals, it merely reported intensity of animal study, notably with terrestrial arthropods or aquatic species greatly overrepresented thus appearing disproportionately high. Such issues require critical re-evaluation and restatement of mainly microbial biota, as is attempted herein.

In 1994 Robert May had assessed ~ 85% of all species as terrestrial (May 1994), and Benton (2001: table 1) extrapolated life on the Land to 12 million species, then being as much as 25× as diverse as in the Sea (just 0.5 million species), i.e., > 96% species on the Land vs < 4% in the Sea. Grosberg et al. (2012: table 1) found most macroscopic organisms were land-based (80%) compared with few in the oceans (15%), and fewer still in freshwater (5%). A recent status paper, like Decaëns et al.’s by Román-Palacios et al. (2022) had only ~ 2 million known species with 80% animals vs 20% plants, plus microbes and fungi needing to be added(!). Claiming combined relative proportions on the Land vs Aquatic of 78% vs 22% (Fig. 2), these authors yet failed to differentiate, making no mention of the Soil nor extrapolating likely totals, again downplaying soil biotic scope.

Figure 2. 

Román-Palacios et al. (2022: fig. 1) summed ~ 1.9 million extant animal and plant species combined, with ~ 80% on Land (~ 1.5 million species) without differentiating those found in Soil. Prokaryotic microbes were not included in their datasets, massively diminishing their terrestrial components (cf. Fierer et al. 2007). Lower still than in Freshwater, speculative claims that the Ocean supports > 80–99% of global biodiversity are readily dismissed by such solidly grounded facts.

Further refinement of these Land vs Aquatic proportions was determined by Blakemore (2022) stating: “Based on topographic field data, an argument is advanced that Soil houses ~ 2.1 × 10²⁴ taxa and supports > 99.9% of global species biodiversity, mostly Bacteria or other microbes. Contradictory claims that Soil is home to only a quarter of biota while Ocean harbors 80–99% of Life on Earth are both dismissed.” This statement requires clarification against Anthony et al.’s (2023) assertion that Soil hosts around 59% of species whereas their tables show only 10–50% (as tabulated below). Halving true proportions, their data totals are underestimated by orders of magnitude, seemingly due to them using older microbial count sources that have now been far superseded (Fig. 3).

Figure 3. 

“Micro monde” progressions with microbial proportions greatly increased from Blakemore (2022, 2023: table 1, fig. 9) after Larsen et al. (2017: fig. 1). Of note, Larsen et al. (2017: tabs 1 and 4) in Scenarios already had Bacteria with ≤ 91% of total at up to 5.2 × 10⁹ taxa, compared to Anthony et al.’s (2023) 4.3 or 10 × 10⁸, these being mainly terrestrial, parasitic, or pathogens related to soil animals.

More than a decade ago, prior to Larsen et al. (2017), a call for a “Census of Soil Invertebrates” (CoSI) catalogued 210,000 known soil species (Blakemore 2012: table 1) itself downplaying most microbes. An updated version had > 315,000 soil organisms (Blakemore 2016), but this also tallied just a sixth of total taxa as then known, albeit with massive proportional unknowns (Table 1).

Regarding Table 1 data, it may be noted that higher organisms host many unique symbionts or parasites and, as for microbes, many specific viruses too. For earthworms, Lee (1985: table 7) had highest numbers in NZ pastures (2,020 m^-2 with 305 g m^-2), higher counts are for littoral Pontodrilus sp. in WA. Note too that a total number of megadrile earthworms was predicted at ~ 35,000 species. Earthworms are important, as is noted later, due to their activities that greatly enhance other soil biota/microbes. Regarded as superficial soil-dwellers, “Soil Gastropods” were tabulated. However, viruses were only provisionally included as they fail to meet all criteria of independently living organisms, albeit they are included in several more recent biodiversity surveys. If such a line of argument for viruses were followed, then may not eukaryotic endosymbionts that only actively exist within host cells (Sagan 1967), with their unique genomes, be similarly added in biodiversity totals?

Contemporaneous to CoSI, a Global Soil Biodiversity Atlas (GBIF 2016: table 1) tallied 219,000 soil fauna/microbes while adding 350,700 vascular plants – on a premise plant seeds and roots are grounded in soil – raising totals to 667,000 soil taxa or roughly a third of all ~ 2 million species formally described as that time (Fig. 4).

Figure 4. 

Global Soil Biodiversity Atlas (GBIF 2016) reporting ~ 667,000 soil biota or just about one third of known 2 million (much above Decaëns et al.’s (2006) 25% total!). Note that earthworms have 7,000 known and > 30,000 estimated species. Bacteria had 15,000 known species but estimated over one million (< 1.5% described). However, when microbes (excluding viruses) are properly considered and counted, as herein, soil unknowns are much higher (likely just < 0.0001% known at best). Vascular plants add ~ 400,000 species (cf. Anthony et al. 2023 with 466,000 angiosperm “Plantae”).

At around the same time, a 10-year, $1 billion, Census of Marine Life (CoML 2010) funded 2,700 researchers at 670 institutions from > 80 nations to conclude a total of just ~ 230,000 Ocean taxa (or ~ 12% of the 2 million known species!). They claimed this was just one tenth of Ocean’s expected total of another two million species, hence a new Ocean Census project “launched” on 27 April 2023 to net the remainder. A similar 2011 Census of Deep Life (CoDL), a central pillar of the Deep Carbon Observatory (DCO - https://deepcarbon.science/), investigated diversity, distribution, and biogeography of obscure subsurface biospheres having little relevance to evolution or extinctions.

As argued in the current report, such expensive sub-marine projects distract funds and efforts from surveys of more crucial soil biota that are much less well-known and more endangered, extinctions being time critical. How is it justified to fund long-term abyssal taxonomy at $ millions per species while unknown soil taxa, that may be easily sampled in the field with a spade, are being extincted?

Although primarily concerned with rapid advances in molecular analyses (“Omics”) revealing microbial diversity increased by several orders of magnitude (as detailed herein), lesser concerns are upping of counts for topographical terrain and delving into soils to full depth. However, unlike routine biotic surveys via planimetrically flat transect, plot, or quadrat, some surface-area independent inventories (e.g., of farm stocks or people) do not gain from realistic terrain extrapolation, neither do level waterlogged entities (e.g., lakes, mires, or bogs).

In general, prior to 2018 almost all soil inventories were based upon unrealistic, planimetrically flat land areas, thus true soil counts are likely more than doubled, and possibly quadrupled, when properly allowing for terrain and microtopography overlays (Blakemore 2018b), reducing further the marine majority claims. Although such work shows Soil is clearly more crucial and diverse, due to lack of equable support or funding, less than 1% of its meso- and macro-faunal organisms are as yet unearthed (FAO 2020). Furthermore, only a tiny fraction of the enormous soil microbiome is identified, with the proportion of known soil microbiota likely much < 0.0001% (as per Blakemore 2023), thus most of the vast array of Soil Biota remain an unexplored mystery awaiting discovery.

Moreover, rather just scratching the surface to cm or a metre deep, recent studies have mean depth-to-bedrock at 13.1 m plus friable saprock may add 8 m to total > 21 m soil depth (Shangguan et al. 2017: table 1; Hicks-Pries et al. 2023; Blakemore 2024). Consequently, most soil estimates, including those herein, may be an order of magnitude too low and hence the relative soil biomass and diversity values as in Fig. 5 (cf. Fig. 2) are most modest. This dependent upon estimates of full soil depth.

Figure 5. 

Global biomass (plus dormant/ necromass) and biodiversity in context of biome proportions (from Blakemore 2023 after https://vermecology.wordpress.com/2020/05/27/realms-of-the-soil/: fig. 2 and https://veop.files.wordpress.com/2022/09/new-addendum-file.pdf: fig. 4), being updated in the current report. In the figures above “below ground” or sub-surface refer to soil biotic activity related to surface productivity and not to the deeper subsurface biota.

Extrapolation of soil sampled at just a few superficial centimetres or a metre, to allow for full depth (≤ 21 m as noted above) are not yet applied but in themselves may increase soil stocks by an order of magnitude. Rolando et al. (2021) found soil layers below 90 cm up to 5 m deep accounted for 80% biomass, while the 0–30 cm layer represented only 10% of total soil carbon (i.e., × 10 for > 30 cm).

A further distinction is definition of “deep subsurface” biota that source energy differently to subsoil species. Beaver and Neufeld (2024) state that there is no universal depth that defines the terrestrial subsurface biome, previous publications having described “terrestrial subsurface as deeper than 8 m, and the deep terrestrial subsurface as deeper than 100 m.” For the purposes of their review, the deep terrestrial subsurface comprised of rocks and groundwater at least 100 m below the surface of the Continents. Bar-On et al. (2018) “define deep subsurface as the marine subseafloor sediment and the oceanic crust, as well as the terrestrial substratum deeper than 8 m, excluding soil.” An important demarcation, although unapplied in the present review, is soil biotic biomass and biodiversity to whole mean depth of soil activity (or frozen in Permafrost), now globally averaged near 21 m.

Soares et al. (2023) suggested 12–20 % of Earth’s biomass exists in the terrestrial deep subsurface, compared to ~ 1.8 % in the deep subseafloor, further confirming “terrestrial deep subsurface holds ca 5-fold more bacterial and archaeal biomass [thus, by proxy, biodiversity?] than the deep marine subsurface.” Although the total Ocean biota is again diminished, this deep subsurface data is a much lesser concern in the current global review of Soil biota and is only briefly mentioned in passing.

Abundance of biota relates to both its biomass (living, dead, or dormant forms) and its biodiversity species counts. Initially, a preliminary global microbial abundance estimated by Whitman et al. (1998: tabs 3–5) was 2.6 × 10²⁹ vs 1.2 × 10²⁹ cells in Soils vs Aquatic (marine and freshwater) habitats, and 26.0 vs 2.2 Gt C biomass, respectively. This was an indicator that Soil clearly supports twice the Ocean biota, and ten times its biomass as an early realization that Soil likely supports > 50–90% of Life on Earth. Deep sub-surface microbiota, which are largely irrelevant to most active above-ground Earth processes, were 3.6 × 10³⁰ vs 2.5 × 10³⁰ cells in Oceanic vs Terrestrial sub-surfaces. However, revisions by Kallmeyer et al. (2012), Parkes et al. (2014), Magnabosco et al. (2018), and Hoshino et al. (2020) had just 3–5 × 10²⁹ vs 2–6 × 10²⁹ cells (biomass of ~ 4 vs 23–31 Gt C), respectively. A global tally of ~ 10³⁰ cells was determined independently by Blakemore (2022, 2023), but for somewhat different proportions, for reasons as explained and briefly restated herein.

Soil was shown with ≤ 10⁸ –10¹² cells/g dry weight or 10¹⁴ –10¹⁸ cells/t, there being 10⁶ grams in a tonne. Biodiversity ranges were 10² –10⁶ species/g or 10⁸ –10¹² species/tonne of soil. Global topsoil was calculated as ~ 2.1 × 10¹⁴ t to 1 m depth. Therefore, the total ranges were 2.1 × 10²⁸ –10³² cells (median ~ 2.1 × 10³⁰) and 2.1 × 10²² –10²⁶ soil species (median ~ 2.1 × 10²⁴). Having a new mean soil depth of ~ 21 m would possibly increase these by an order of magnitude, but is not yet applied. Comparatively, Anthony et al.’s (2023) global species total (10¹⁰) is mid-range in the biodiversity of a single tonne of topsoil. Moreover, an equivalent to all the Oceans’ biodiversity may similarly be held in just a handful of fertile topsoil, or much less than a tonne, albeit, as a general “rule of thumb”, a dry tonne of topsoil occupies 0.65 cubic metres, a ground area of < 1 m², or a small step for a man.

Prior sources had determined: “species of bacteria per gram of soil vary between 2,000 and 8.3 million” (Gans et al. 2005; Roesch et al. 2007) (= 10⁴–10⁶ spp/g or 10¹⁰–10¹² spp/t that, if all unique taxa, is equivalent to twenty billion or up to a trillion species per topsoil tonne). Discrepancies in Gans et al. (2005) are samples of 10 g soil so strictly 0.83 × 10⁶ spp/g, yet their fig. 4 shows total species number computed as ≤ 10⁷ thus a million or so spp per g seems correct. Raynaud and Nunan (2014) had: “a single gram of soil can harbour ≤ 10¹⁰ bacterial cells and an estimated species diversity of between 4 × 10³ to 5 × 10⁴ species” (= 10¹⁴–10¹⁶ cells/t and 4 × 10⁹–5 × 10¹⁰ spp/t). Bickel and Or (2020) found: “bacterial phylotypes ranges between 10² and 10⁶ per gram of soil, with high values similar to the diversity in all of earths environments” (= 10⁸–10¹² spp/t). James et al. (2022) summarized: “Soil microorganisms are the largest biodiversity pool on earth, with more than 10³⁰ microbial cells [total surely!?], 10⁴ –10⁶ species, and nearly 1,000 Gbp of microbial genome per gram of soil”. Although fully extrapolated values from the cited reference sources are listed, only the median of the various value ranges are taken as reasonable summaries, these being presented herein.

As already noted, using scaling values, Zhao et al. (2022) found: “Although the estimated total abundance of global airborne bacteria (1.72 × 10²⁴ cells) was 1 to 3 orders of magnitude lower than that of other habitats, such as soil (9.36 × 10²⁸ cells), freshwater (4.70 × 10²⁵ cells), and marine (4.68 × 10²⁸ cells) habitats, estimates of the bacterial richness of the atmosphere (4.71 × 10⁸ to 3.08 × 10⁹) were comparable to those of the hydrosphere”. In other words, they confirm Soil at ~ 10²⁹ with twice as many microbial cells as the Ocean and, whereas their figure (Zhao et al. 2022: fig. 3A) shows a richness of > 10¹¹ soil microbe OTUs, the Ocean or Freshwater and the Air each only have ~ 10¹⁰ taxa (< 10%). This translates as Soil housing ~ 90% of global biodiversity, as indeed May (1994) had intimated 30 years ago, before the scope of microbial megadiversity was realized as being so vast.

For microbial diversity, recent developments of rapid genomic sequencing and bioinformatics (-omics) allow scaling values such as by Locey and Lennon (2016) to show Earth with ~ 10¹² microbial OTU taxa (just 10¹⁰ or ~ 1% in global Ocean). These totals were soon raised to 10¹²– 10¹⁴ microbial taxa by Lennon and Locey (2020) and then by Fishman and Lennon (2022) who had “a soft upper constraint of 10²²–10²³ due to neutral drift” for all taxa. Their upper boundary is increased by 20× for median species total in the current study and, regardless of scaling values, confirm Soil’s > 99.9% of global biodiversity, being almost entirely microbial. These authors’ soft upper constraint of 10^22–23 taxa dispersed in 10^29–30 soil cells is a ratio of one taxon per ~ 10^6–8 cells.

Summarizing the microbial status, Zhao et al. (2022) said: “soil is the most microbiologically abundant (10²⁹) and diverse (10¹¹) environment on the Earth”. Although their cell count may be within bounds, their diversity – albeit ~ 10× greater than the Ocean’s – is disproportionately low due to incomparability of Soil’s scaling ratio when compared to any of their other habitats (Fig. 6; Table 2) .

Figure 6. 

Relative microbial abundance vs diversity after Locey and Lennon (2016: fig. 3), and Zhao et al. (2022: fig. 3A) who added wastewater, air, freshwater, and soil. The Ocean has < 1% of global biodiversity, barely above freshwater or air, and less than the human gut biome! The soil microbiome is revised upwards in Table 2 as its abundance vs richness apogee peak is more extensive than any other major (or minor) habitat.

Table 2 contextualizes a current estimate of 2.1 × 10²⁴ soil taxa as 99.99% of a global total. Species values presented herein (e.g., Fig. 6, Table 2) may be contrasted to microbial Spp/OTU counts in Anthony et al. (2023: table 1), arranged in a slightly revised format for better clarity of comparison, as shown in Table 3.

Deep carbon data are of less practical concern to the current study on Land and Soil carbon stocks and cycles, although they again highlight deficiency of Ocean’s excessively claimed biota at all scales and at all depths, almost all being downgraded in subsequent reviews.

Anthony et al. (2023: fig. 2) confirm Bacteria richness ≤ 90%, proving their dominance in Soil (Fig. 7) but are mistaken for Oligochaeta, as Earthworms are truly higher with 99% soil occupancy which as is discussed further in the review section below.

Figure 7. 

Unsystematically selected taxonomic groups in Anthony et al.’s (2023: fig. 2) (cf. Table 1) appear to show Bacteria’s Upper dominance at > 90% in both Soil and “Global (no phage)” totals but, strangely, they omit Megadrile earthworms being > 99% resident in soils, as their name would suggest and as restated below. Comparatively, their Enchytraeidae, according to Martin et al. (2008), are < 47% terrestrial, not 99%!

The intention of this review is to compile and compare recent Soil Biota studies by Blakemore (2022, 2023) with Anthony et al. (2023). In the last few decades, the advent of high-throughput DNA amplicon sequencing and rapid genetic analyses (-omics) has revealed the complete dominance of microbes in biotic tallies, especially in soils, and a need for realistic biodiversity estimation from projections of their unknown and undescribed components. Realizing our ignorance of soil microbes exposes a stark disparity: Most accounts of global richness reflect historic intensity of study rather than relativistic estimates due to irrational fact of overwhelmingly research effort and funding directed into Aquatic, Oceanic or Space research (e.g., NASA, JAMSTEC, NOAA, Scripps, Woods Hole, https://en.wikipedia.org/wiki/List_of_oceanographic_institutions_and_programs), not Soil. The International Union of Soil Sciences (https://www.iuss.org/) does not list any dedicated institute.

The summary of progress in relative soil biodiversity studies, as introduced above, is further reviewed and where necessary corrected, mostly for microbe counts but also to allow for terrain (after Blakemore 2018b). Factoring soil depth may further double numeric values if not exponentials.

In addition, several omissions and uncertainties (“?”) from various published sources are clarified.

Shortcomings in Decaëns et al. (2006) soil biodiversity summary as (shown in Fig. 1) are mainly that it only reports intensity of study, not estimated totals, and mostly ignores microbes that have since become paramount. Other flaws in its premise are that since around two million species had already been described at that time (MEA 2005: Chapter 4), then their 23% in 360,000 species would likely have been closer to ~ 18%. Conversely, if their > 23% in Plants, Fungi, Bacteria, and viruses that are all mostly found in soil were added to their 23% total, the soil proportion is doubled, being raised to > 46%, albeit only ~ 1% of soil organisms are known (FAO 2020). Hence, would a likely new total for their study if 100 × and of around~ 38 million species not already have a 99% majority in soils?

Moreover, it appears that estimated soil fungi alone supported as many as their claimed global total of 1.5 million species: “The estimated global fungal diversity has changed dramatically from 100, 000 in the 1940s to 1.5 million in the early 2000s, then 2.8 to 3.8 million in the 2017s, and currently 2.5 million species as the best estimate. However, 155,000 species are currently known; thus, many species are still undescribed and waiting for their discoveries” from https://mycokeys.pensoft.net/topical_collection/254/. These known fungi should also be in soil total.

Anthony et al. (2023) claimed their 1.04 × 10¹⁰ soil species estimate as “approximately two times greater soil biodiversity than previous estimates” but it was considerably less than Zhao et al. (2022: fig. 3A) already with 10¹¹ soil microbial OTUs that was revised upwards to 2.1 × 10²⁴ species by Blakemore (2022). Both prior studies surpass their subsequent 2023 findings indicating a need either for rebuttal or for a thoroughly refined restatement of both local and global soil biotic enumerations.

This review of vital Soil Biota aimed to clarify its true scope while indicating key areas in need of understanding. The vast array of faunal, floral, fungal, and microbial groups and their roles are mostly unexplored and open for investigation, emphasizing an urgent need to establish a Soil Ecology Institute. Until this is fully realized, in the interim, myriad Aquatic or Atmospheric facilities abound, although the naturally depauperate Ocean and void Space will mostly remain intact regardless as they do not erode, neither do they flood nor burn. Ocean issues are solved in Soil. Due to the most pressing problem of topsoil erosion and irreversible extinction losses, a major shift should be realizing the overwhelming importance and fragility of our precious Soil. The need for proportionate fund reallocation (hence no extra costs involved) to support urgent and directed soils research – under the principles of true Context with systematic Triage – to benefit all Life on Earth.

A supporting homage to our origins and reliance on Microbes in Soils is a diagrammatic Tree-of-Life, as alluded to in the Abstract and Introduction, showing common microbial ancestry origins and prehistoric extinction events – https://web.archive.org/web/20240705043415/http://evogeneao.s3.amazonaws.com/images/tree_of_life/tree-of-life_2000.png. The author notes that this is a phylogenetic tree not reflecting biodiversity.

This Tree-of_Life is particularly poignant with regards to a mostly mysterious soil virome as expounded by Paez-Espino et al. (2016). Williamson et al. (2017) discuss similar issues, coming to a simple conclusion: “Soils remain the most poorly understood ecosystems on Earth. At the same time, viruses represent the largest pool of untapped genetic diversity and unexplored sequence space on the planet. In this regard, the soil virome comprises an unknown quantity within an unexplored territory: a vast new frontier, ripe with opportunities for discovery.” The current report is not alone in realizing such magnitudes, nor in urging for more support for Soil Eco-taxonomic restoration in order to boldly explore this vast new frontier lying in wait directly beneath our feet, while it still exists.

While focusing on fundamental soil microbiome, it is important to note this is enhanced by activities of a literal ground-breaking master of its domain as manifest in Darwin’s “humble earthworm”.

Promoting earthworm activity, as advocated by Blakemore (2018a, 2022, 2023), increases plant growth and provides microhabitats for soil fauna and flora, viz.: “microbes increase during digestion and after gut passage in their fresh castings by up to × 1,000 (Lee 1985: 27, 206) further enriching soils.” Presumably the viral abundance is also increased by a multi-fold magnitude due to such actions. Thus, a simple solution to soil degradation is to attempt, in any way and at all times, to preserve and enhance earthworm populations that are more accessible than microbes. As Bill Mollison, co-founder of Permaculture and author of their Designers Manual (Mollison 1988) said: “There is one, and only one solution, and we almost have no time to try it. We must turn all our resources to repairing the natural World, and train all our young people to help. They want to; we need to give them this last chance to create forests, soils, clean waters, clean energies, secure communities, stable regions, and to know how to do it from hands-on experience.” (https://web.archive.org/web/20240719045254/https://www.azquotes.com/quote/873849).

That the Soil hosts > 99.9% of global diversity now requires a major “Sea change” in attitudes and funding to recognize its true scope. This should spur formation of at least one dedicated Soil Ecology Institute (for both natural and managed lands) tasked to catalogue, research and reverse mass degradation of our planet’s most crucial, yet most neglected ecosystem – that of the Soil Realm.

Dave Loneragan and Rose Andrews of Kangaroo Valley and Rowan and Robbie of Berry, NSW kindly provided accommodations during formulation and compilation of this review. Constructive critiques of editors and referees are appreciated in helping improve the paper and, although some of the enforced edits detract, I take responsibility for any unintended errors and oversight omissions that remain. The authors of Anthony et al. (2023) soil enumeration paper were emailed for critical comment and respectful collaboration in 2023, but they have yet to respond.

Permission for use of previously published images was sought from original authors as cited, or is compliant with Proceedings of the National Academy of Sciences (PNAS) and GBIF allowed copy.

Biomass of earthworms and microbes, as major soil organisms in the various biomes, are compared to minor microdriles (viz. Enchytraeid potworms that were given inordinate importance by Anthony et al. (2023)). These data are presented in Table A1:

Earthworm biomass at 3.8 Gt C and Microbes at 209.6 Gt C are slightly higher than 2.3–3.6 Gt C and 200 Gt C, respectively, as estimated by Blakemore (2023: table 2) confirming importance of both groups to Soil Ecology.

This new earthworm value of 3.8 Gt C may be doubled to ~ 7.6 Gt for dry biomass which is substantially higher than the 0.9 Gt (and thus 0.45 Gt C?) as lately reported in Miller et al. (2024). Their paper overlooked 4.5 Gt dry and 2.25 Gt C data in Blakemore (2017 -https://vermecology.wordpress.com/2017/02/12/nature-article-to-commemorate-charles-darwins-birthday-on-12th-feb/) independently extrapolated from the extensive works compiled by the leading earthworm ecologist and taxonomist, my PhD assessor and mentor, Dr Ken Lee (1985).

Virus-Like Particle (VLP) counts (Global and in Soil alone)

All viral estimates in Anthony et al. (2023: table 1) had speculative uncertainty marked “?”. Indeed, Williamson et al. (2017) found soil viral diversity severely underestimated and under-sampled, albeit their measures of viral richness were much higher for soils than for aquatic ecosystems. Many soil virus reports show ≤ 10¹⁰ –10¹¹ virions per gram of soil and a global best-estimate tally was of > 10³¹ virus-like-particles (VLP) that are infecting microbial populations at any one time (Mushegian 2020 originally cited from Hendrix et al. 1999).

If 10¹⁰ –10¹¹ virions per g occur, this is 10¹⁶ –10¹⁷ per tonne of soil. Further, if there exist 2.1 × 10¹⁴ t topsoil to 1 m depth (from Blakemore 2022), then a range is 2.1 × 10³⁰–10³¹ virions (with a median value ~ 1.5 × 10³¹).

This total is approximately the same as that calculated by other authors, e.g., Mushegian (2020) or Cobián-Güemes et al. (2016) of between 10³¹ and 4.8 × 10³¹ viral phages. Comparatively, Suttle (2005) extrapolated counts of Marine viruses from local samples to the entire World, arriving at an estimate of 4 × 10³⁰ virus particles in well-mixed oceanic waters (i.e., ~ 10%). Mushegian (2020: table 1) arrived at a similar estimate to this at 2 × 10³⁰ virions in the Ocean, i.e., a range of just 2–4% of Ocean virions in a global total of ~ 10³¹ virions.

Previous range of Ocean virus proportions is then just 2–10% of global totals with much of the remainder (90–98%) in Soil. True Soil counts are variable, as shown below, further reducing the proportional Ocean values.

While initial estimates of virus abundance in Soil ranged from 10⁷ to 10⁹ virus like particles (VLP) per gram of dry soil (Williamson et al. 2005, 2017) with a mean ~ 10⁸, Pratama et al. (2020) found a higher mean of 10⁹ VLP per g from a range of catalogued soils and Graham et al. (2023) had 10⁷ to 10¹⁰ viruses per g of soil. This agrees with Jansson (2023) for different soil types at 10⁸–10¹⁰ VLP per g dry soil (mean also 10⁹), but she noted the true number may be higher than that obtained by microscopy because many soil viruses are intracellular and not able to be imaged separately. Kannoly et al. (2022) shows ≤ 1,430 plaque-forming-units (PFU) per lysogenic bacterial cell-burst (a so-called “burst size” of viral particles). Thus, an exponential order or two is easily added, possibly to allow reasonable estimates of around 10¹⁰–10¹¹ VLP per g of dry soil?

Especially relevant, a study by Cobián-Güemes et al. (2016) estimated 4.8 × 10³¹ VLPs on Earth but comprising an unrealistic minimum of 257,698 different viral genotypes (sic). They quoted reports with only 3.9 × 10⁶ ≤ 2 × 10⁹ total varieties (or one variety per 10²²–10²⁵ VLP), which seems a wide underestimation compared to other studies. Their VtB ratios (in their table 1) were skewed by a low “Human associated” ratio of just 0.1 and a high “Other host-associated” of 25, to give a median ratio for all their biomes of 12. Their Soil VtB was 19.1. Here revised microbial counts in Blakemore (2022, 2023) give a global total of > 5.1 × 10³¹ VLPs with ~ 4.1 × 10³¹ (~ 80%) virions in soils (to full depth?) and a Soil alone VtB near ~ 20:1, as in Table A2:

Whitman et al.’s (1998: table 5) 3.6 and 2.5 × 10³⁰ cells in Oceanic or Terrestrial sub-surfaces were downgraded by Kallmeyer et al. (2012), Parkes et al. (2014), Magnabosco et al. (2018) and Hoshino et al. (2020) to just 3–5 and 2–6 × 10²⁹, or ~ 4 × 10²⁹ each, and with sub-surface biomass of 4 and 23–31 Gt C, respectively. Bar-On et al. (2018: supp: 62) for Marine, Sub-Ocean, and Sub-Soil, had 1.2 × 10²⁹, 4 × 10²⁹ and 20 × 10²⁹ cells, respectively. For Soil, their Prokaryote total was ≈ 3 × 10²⁹ cells, similar to the values presented herein.

The mean VtB ratio 11.4 is approximately the same as Cobián-Güemes et al. (2016: table 1) median VtB of 12, both above Bergh et al. (1989) ~ 10:1 aquatic VtB. If 11.4 is applied to 302 × 10²⁸ Microbes a total is ~ 5 × 10³¹ VLPs. However, Soil alone VtB is double at ~ 20:1 that, for 210 × 10²⁸ Microbes, is ~ 4 × 10³¹ VLPs (~ 80%). Thus, proportion of viruses in Soil from total viruses is ~ 80% as noted in the Abstract (cf. Ocean 2–10% noted above), with the remainder in the deep-subsurfaces.

Although most samples are superficial, often in just the top 5 or 10 cm of soils, viral activity persists throughout the soil profile, to at least 1 m depth according to Muscatt et al. (2023). Interestingly, these latter authors stated: “Viral contributions to soil ecology are largely unknown due to the extreme diversity of the soil virosphere. Despite variation in estimates of soil viral abundances (10⁷ to 10¹⁰ viruses per gram of soil), it is clear that soils are among the largest viral reservoirs on Earth. Early metagenomics investigations have revealed high genetic diversity in soil viruses, with putative impacts on global biogeochemistry. Still, less than 1% of publicly available viral metagenomic sequences are from soil, reflecting the lack of knowledge about soil viruses and their ecological roles”. Accordingly, as soil is so under-represented, Graham et al. (2023, 2024) argue that understanding the rôle of viruses in soil is most pressing of any of our ecological challenges.

Virus to Bacteria (VtB) ratio abundances

As already noted, Mushegian (2020) had an approximate 10-fold excess of phages over bacterial cells (as per Bergh et al. 1989), whereas Cobián-Güemes et al. (2016: table 1, fig. 1) median VtB was around 12:1 and in Soil alone ~ 20:1 (or 100:1 mean soil ratio in their figure which is an order higher that all other ratios). Applied to Table A2 totals, a 12:1 ratio for all ~ 3.78 × 10³⁰ global microbe cells would be ~ 4.6 × 10³¹ VLPs with 4.1 × 10³¹, or 89%, of viruses in Soil. But this too may be out by an order or more, not least to account for those active intracellular virus particles that are mostly overlooked in general surveys, or in the Soil’s VtB ratios.

Early on, Ashelford et al. (2003) averaged soil virus numbers at 1.5 × 10⁸ per g, which they said was equivalent to 4% of total bacteria population (of 3.6 × 10⁹ per g) giving a virus-to-bacterium ratio (VtB) in their soil of 0.04:1. Subsequently, other authors found much higher numbers, e.g. Cobián-Güemes et al. (2016: fig. 1) had mean Soil VtB ratio ~ 100:1 but selected a median value of ~ 20:1 as in their table 1 (cf. Table A2).

Cao et al. (2022) reported highly variable virus-to-bacteria ratios (VtB) in soils as ranging from 0.001 to 8,200 (six orders of magnitude!) although their study found abundance of virus-like particles (VLPs) ranged from 2.0 × 10⁷ to 1.0 × 10¹⁰ and microbial abundance ranged from 1.0 × 10⁸ to 8.2 × 10⁸ per gram of dry soil, to give a VtB ratio from ~ 0.1 to 98.3 (near three orders of magnitude range), settling around a median VtB ratio value of 10:1 compliant with Cobián-Güemes et al. (2016: table 1, fig. 1) but including depauperate aquatic biomes. Wide ranging (VTM/VBR = VtB) estimations, pertinent for soil, mostly vary ~ 10:1 to 100:1 which is interesting as this complies with a virus species to host species range as assumed by Koonin et al. (2023).

VtB ratios (= VTM/VBR) for species diversity

Although an answer is complex, a preliminary estimate in Fierer et al. (2007: table 3) had bacterial OTUs of 10^3–6 (median ~ 5 × 10⁴) while viral vOTUs ranged 10^3–8 (median 10⁶) thus, extrapolating data, soil viruses may appear 10–100 times more diverse than Bacteria as a rough indication of mutual biodiversity crosscheck. As just noted, Koonin et al. (2023) conservative range estimate was 10:1 to 100:1 for their host species ratio. Muscatt et al. (2023) determined: “overall vOTU per host ratio was 0.42 (median = 0) [sic], reflecting the predominance of unique host associations for individual vOTUs”. This suggests viral diversity is commensurate with Bacteria/Archaea diversity (vOTU:bOTU) and vice versa. So, for 2.1 × 10²⁴ soil microbe species, viral diversity would be at least (2.1 × 0.42 = 0.88) or around 0.88 × 10²³ vOTUs. Viral richness (~ 10²³ per 10^31–32 virions) would then be ~ 1 unique ‘variety’ for each 10^8–9 virions, or roughly two or three orders lower than bacterial richness which, as noted, is around one bacterial taxon per million cells. Q.E.D.

Conversely, Roux and Emerson (2022) quote: “estimates of soil viral richness suggested the presence of 1000 to 1 000 000 genotypes per sample” and samples were traced as ~ 200 g wet soil, say ~ 100 g dry, to give around 10–10,000 soil genotypes per gramme (or around 10³ in a mean of 10⁹ virus particles per g which is also ~ 1 vOTU per million virion cells as with bacterial estimates). Thus, both are at a mutual 1:1 ratio. Recently, Graham et al. (2024) quoted 10⁷–10¹⁰ viruses per gramme of soil showing soils as the largest viral reservoirs on Earth and they reported averages of 40.01 (range 1–2,124), 35.48 (range 1–1,651) and 24.91 (range 1–896) unique viral clusters per soil sample (presumably 1 g?) at the species, genus, and family levels. This suggests four viral species for each one million, up to a billion soil virions, a bit higher than for Bacteria.

A likely summary is that viruses are most abundant in soils and at least ten or 100 times as rich as the Bacteria, their primary hosts. From Blakemore (2023), as both Global and Soil alone bacterial biodiversity are in the order of 2 × 10²⁴, then virus diversity may range from at least as many, ≤ 10²⁵–10²⁶ viral species in total.

Alternatively, as ~ 10³¹ viruses are known, may soil Bacteria reasonably range 10²⁹–10³⁰ species?

Support is found in Kuzyakov and Mason-Jones (2018), viz.: “The total number of viruses (including intracellular viruses inside bacteria) is probably 1–2 orders of magnitude higher than the bacterial populations” From this we may again conclude in circular argument that Bacteria are 1–2 orders less in terms of both cells and species.

Finally, we may concur with Williamson et al. (2017): “To understand the soil virome, much work remains.”